It is known to use controlled thyristors, such as SCRs, in the secondary circuit of polyphase AC wound rotor motors for controlling the secondary impedance. However, their use has not gained acceptance for industrial uses in hoist drives and the like. A basic problem resides in providing stable AC reference waveforms that will be correctly timed phased to the fundamental while being insensitive to electrical noise and motor harmonics. Traditional approaches that rely on a fixed frequency AC reference using active or passive filtering techniques cannot be satisfactorily applied to the type of AC wound rotor motor secondary control.
The most suitable approach heretofore used, comprises coupling an auxiliary machine, such as a three phase synchro, directly to the shaft of the controlled motor. While this method is straight forward in developing a suitable AC firing control referenc it has a cost disadvantage and requires precise mechanical alignment with the controlled motor to assure proper synchronization of the firing gate current to the thyristors.